The present invention relates to a multicolor image-forming method and multicolor image-forming materials to be used in performing the method. The materials of the present invention are particularly suited to a method for preparing a prepress color proof capable of being used in the course of proof work indispensable to a photomechanical process and materials to be used in performing the method.
The prepress color proof is used for the purpose of checking color, tone, etc. to be obtained after printing at a stage just before the plate-making, that is, at a stage of making or using a screened color separation positive or negative before processing the plate, similar to a proof sheet made customarily in color printing by photographic means. With regard to the preparation of this prepress color proof, there have been known a Surprint method, overlay method and transfer method hitherto as principal methods according to their tyes of color proof making.
Surprint method is a method to form a multicolor image, wherein colored photosensitive solutions in each color are coated in turn onto a sheet of white substrate and printing is repeated using screened color separation positives or negatives. Overlay method is one for proofing by superposing prints, wherein printings in each color are made on transparent substrates coated with colored photosensitive agents using screened color separation positives or negatives. Also, transfer method is one, wherein a multicolor image is formed in turn on a white sheet by means of a transfer process by pressure-sensitive adhesive and transfer by heat.
Among these, however, labor and time are required in performing the Surprint method, since printings are made as the photosensitive solutions in each color are coated in turn. Also, in the overlay method, turbidity is caused resulting in a considerable difference in color compared to the print obtained with printing machine or proof press, since each transparent substrate is superposed. In this respect, the transfer method, wherein a closely related print to the final one is obtainable and yet the procedure is relatively simple, has attracted a great deal of attention. Nevertheless it has problems as follows.
Namely, in the method a pressure-sensitive adhesive, colored photosensitive layer, pressure-sensitive layer and backing layer (release paper coated with releasing agent) are principally layered on the substrate in order. As a sticky surface appears by removing the backing paper, this is contacted closely enough under pressure with white-base mounting paper to be transferred. The substrate is removed to transfer the colored photosensitive layer to the mounting paper. Then, the colored photosensitive layer is exposed to light in close contact with the screened color separation positive or negative having a corresponding color tone to said colored photosensitive layer, developed with the exclusive developing solution, and dried to obtain a separated colored image. With regard to the other colors, the same procedure is further repeated on the image obtained as above to form a multicolor image. Since the constitution of the color sheet for the transferring in said process is complicated labor and time are required to form the multicolor image and the material is relatively expensive. Moreover, since the image formed previously is hidden in the color-processed layer on the transferring of second and subsequent colors after the colored photosensitive layer was transferred, there are also such shortcomings on exposure as discrimination being difficult and matching, or registration of image being hard to do.
Also, there are many proposals including a method where transferring is carried out by means of heat in place of pressure-sensitive adhesive. For example, a method (U.S. Pat. No. 3,203,805), wherein a light-exposed area is made to not exhibit stickiness if exposure is made through the a screened color separation positive or negative in close contact with one provided with a colored photopolymerizable layer which exhibits stickiness over a fixed temperature on the substrate, the unexposed area alone is transferred by heat to the paper base, etc.; a method (U.S. Pat. No. 4,304,836), wherein after is exposed and developed the colored photopolymerizable layer on the substrate, the colored image is transferred by heat to a base having thermosensitive adhesive; a method (U.S. Pat. No. 3,721,557, Japan Patent Publication No. 15326/1971, Japan Patent Publication No. 441/1974), wherein after exposure and development of substrate/release layer/colored photosensitive layer to obtain a colored image, the thermosensitive adhesive (in this case pressure-sensitive adhesive may also be used) is coated onto said image surface or the surface to receive the image and the heat transferring is conducted; a method as published in Japan Unexamined Patent Publication No. 41830/1972, wherein a backing layer having a colored image formed by the exposure and the development of a material consisting of substrate/heat-fusible and bondable backing layer for colored image/assistant fixing layer for colored image (heat-fusible and bondable property is not necessarily indispensable)/colored photosensitive layer (not limited), is fused and bonded mutually to a transfer surface and then the colored image is transferred by heat together with the backing layer for colored image by releasing and removing the substrate, or the like.
The constitution of these methods is relatively simple and registration of image between different colors is easy because the image is transferred. However, all of them have a material with a heat-adhesive property only on one side, that is, on either an image surface or the surface of a transfer material. Accordingly, defective transferring is apt to occur and the bonding of the colored image is insufficient to the transfer surface. Moreover, since a fairly high temperature is required to obtain complete transferring and bondability, deformation of the image by heat, dimensional change in the image surface provided the transfer substrate is a plastic film, etc., and the like can occur leading to undesirable results such as the occurrence of discrepancy of the image, etc. Also, such means where the adhesive is applied afterwards, etc. are disadvantageous because of the complicated procedure, the requirement of fairly extensive labor and the lack of reproducibility.
In view of above-mentioned points, the inventors have studied diligently to find a multicolor image-forming method and materials to be used in performing said method having many advantages and applicable also to the color proof method. Namely, the inventors have adopted the transfer method, in particular, the heat transfer process method, among the processes described above, to develop a method which is capable of making up for the defects which are unavoidable by the above-mentioned heat transfer processes. The advantages are shown in that the transferring is complete and easy not only of the first color but also of the second and subsequent colors, that the bondability of the image to the transfer surface is excellent after heat transfer, that deformation of image, the dimensional change, etc. are absent and the matching of the image between different colors is easy as well as complete because of the capability of transferring even at a relatively low temperature through the adequate selection of thermosensitive adhesive (heat-fusible and bondable substance), that simplified proofing by the overlay method is possible at a stage before transferring, that an inexpensive photosensitive material can be used, that the environmental aspects and the safety problems such as explosion, etc. are favorable because of the capability of carrying out all of the treatments in the a lighted room and also carrying out the developing treatment with water alone, that the constitution of the material is simple and the possible savings are great not only in the material cost but also in the production cost, and the like.